The distance of the earth from the sun is such, that the solar rays may be supposed to arrive at the earth's surface in a state of parallelism. Now, when parallel rays fall upon a globe, it is obvious, that any number of such rays falling perpendicularly, as at the equator of our earth, will occupy a very different portion of the surface of the globe; from what an equal number of the same rays will occupy, where they fall obliquely, as in our polar regions. Hence, as we recede from the equator towards each pole, heat and light are diffused, over gradually increasing portions of the earth's surface; and thus the intensity of both decreases in a like proportion. The exact law of such decrease is well known to mathematicians, but need not be here repeated. For our present purpose it is sufficient to observe, that among the natural causes affecting the distribution of heat and light in different latitudes, the globular figure of the earth is the principal.

The second great natural cause of the unequal distribution of heat and light over the earth, is the obliquity of the earth's motion in its orbit, with respect to the plane of its equator. From this obliquity it happens, that, during the annual revolution of the earth round the sun, every part of its surface, between the latitudes of 23½° north and south from the equator, is in turn exposed to the perpendicular influence of the sun.